[Chan Tran]

I try to preset for each situation too. It is very valuable. In fact I started a thread about it today http://www.apug.org/forums/forum48/1...nting-bit.html

So i have a question, what ISO?

It's an honest question, not flippant. It takes all three.

Time and aperture are used to make creative choices, time controls blur, aperture controls DOF.

ISO finishes the equation to get proper exposure.

Im trying to figure out why.

Although I can't reply for cliveh, when I think that the scene looks like it's 1/125 @ f/8 I think of ISO100. And then if I do not use ISO100 I would have to convert or if I don't want to use f/8 or 1/125 I would have to convert too. When I came up to a scene I tend to think it's 1/125 @ f/8 rather than EV13. I would have to use the method described by the OP to convert it to an EV value for ISO100. Taking the above example it's 1/125=7 + f/8=6 so it's EV13 at ISO100. If I am using ISO400 then it's 13+2=EV15. Then If I decide to say use f/11 and that's 7. 15-7=8 and that's 1/250.